All responses

Tags are labels. They works the same regardless of the name. Type @something within the task name to set one up.

Todoist indeed does not sync with iOS reminders, but you get awesome Todoist apps for many platforms including iOS and can have everything in one place and not spread across multiple apps.

Todoist completely consists of checklists ;-) Every task can be checked off. If you create a sub-task, you can have a list of things to do to complete this task. You can have sub-tasks on 4 indent levels. You can also have sub-projects if a big projects requires separate lists below it.